Many products are susceptible to putrefaction, denaturation, mold growth, spoilage, rancidity, oxidation, or other deterioration when brought into contact with oxygen. Examples of such products include beer, wine, juice, vinegar, sauces, seasonings, processed foods, bread, produce, meats, and certain pharmaceuticals and chemicals, among a variety of others. Preservation of such products is disturbed when molds, bacteria, and other organisms that thrive in the presence of oxygen are present. These organisms cause the putrefaction and change in the taste or quality of the product. In addition, some of the products themselves are liable to be affected by oxidation that changes the taste or quality of the product. To prevent such oxidation and growth of organisms and thus increase the preservation stability of these products, the oxygen must be removed from the container in which the products are stored.
One technique for avoiding or reducing the presence of oxygen is vacuum packing. This involves evacuating a container after charging it with the product.
Another technique is gas flushing or displacement. Here, an inert gas such as nitrogen is used to displace the air and hence the oxygen in a container. The displacement can be performed before or after the product is charged to the container.
Still another technique is a foaming method. Particularly applicable to products such as beer, a jet foamer can be used to inject a small amount of pressurized water to foam the beer after charging it to the container. The foam acts as a mechanical deoxygenizer.
Common disadvantages associated with all of the above techniques are the requirement of large-scale apparatus and operation and the difficulty of removing oxygen dissolved in the product. Also, in general, these techniques leave between 0.2% and 5.0% of the oxygen in the container. This amount of oxygen in the container is enough to adversely affect many products.
A simpler, more efficient technique for oxygen removal involves placing an oxygen absorbent in the container with the product. For this purpose, it is known to dispose an oxygen absorbent within a resin that is solid at room temperature. For example, in U.S. Pat. No. 5,143,763, compositions are disclosed having an oxygen absorbent disposed in a resin such as polyethylene, polypropylene, and ethylene-vinyl acetate copolymer, among others. U.S. Pat. No. 5,089,323 discloses compositions having an oxygen absorbent contained in thermoplastic resins such as low-density branched polyethylene, high-density polyethylene, propylene homopolymers, and copolymers of ethylene and vinyl acetate, among others.
Because the resins in these examples are solids at room temperature, application of the resin-oxygen absorbent mixture is often difficult. Accordingly, the '763 reference suggests dissolving the resin in a solvent to form a resin solution to facilitate application of the mixture. Specifically, the processes of forming a solution having an oxygen absorbent in it and applying it by screen printing are disclosed in the '763 reference.
Several limitations are inherent, however, in the process of applying a solution by screen printing. In order to print the solution, the screen must be pressed against the substrate upon which the solution is to be printed. When the screen is lifted to move to another print location on the substrate, the surface tension and viscosity characteristics of a solution are such that there is not a clean, complete separation between the solution that has been printed on the substrate and the solution remaining on or behind the screen. This results in a poor quality print that is difficult to control.
Known oxygen absorbing formulations have other limitations: they can contain only a limited amount of oxygen absorbent, they have limited oxygen permeability, and they are only effective to absorb oxygen in high humidity environments.